Compound optical circuit switch

ABSTRACT

A compound optical circuit switches and methods are disclosed. Two or more 1 st -tier switches may be coupled to one or more 2 nd -tier switches. Each of a plurality of input ports may be connected to a respective input of one of the 1 st -tier switches and each of a plurality of output ports may be connected to a respective output of one of the 1 st -tier switches. Each connection between an input port and an output port connected to the same 1 st -tier switch may be made within the 1 st -tier switch. Each connection between an input port and an output port connected to two different 1 st -tier switches is made along a respective connection path through the 1 st -tier switch connected to the input port, through a selected 2 nd -tier switch, and through the 1 st -tier switch connect to the output port.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND

1. Field

This disclosure relates to optical communications networks and moreparticularly to optical circuit switches using MEMS(micro-electromechanical system) mirror arrays.

2. Description of the Related Art

Communications networks commonly contain a mesh of transmission pathswhich intersect at hubs or nodes. At least some of the nodes may includea switching device that receives data or information arriving at thenode and retransmits the information along appropriate outgoing paths.

Optical fiber links are commonly used to provide high bandwidthtransmission paths between nodes. Such optical fiber links form thebackbone of wide area networks such as the Internet. Optical fiber linksare also applied in high bandwidth local area networks which may beused, for example, to connect server racks in large data centers or toconnect processors in high performance computers.

An optical circuit switch is a switching device that forms connectionsbetween pairs of optical fiber communications paths. A typical opticalcircuit switch may have a plurality of ports and be capable ofselectively connecting any port to any other port in pairs. Since anoptical circuit switch does not convert information flowing over theoptical fiber communication paths to electrical signals, the bandwidthof an optical circuit switch is essentially the same as the bandwidth ofthe optical communications paths. Further, since an optical circuitswitch does not convert information into electrical signals, the powerconsumption of an optical circuit switch may be substantially lower thana comparable conventional (i.e. electronic) switch.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical schematic diagram of an optical circuit switch.

FIG. 2 is a block diagram of an environment for an optical circuitswitch.

FIG. 3 is a block diagram of an optical circuit switch.

FIG. 4 is a block diagram of a compound optical circuit switch.

FIG. 5 is a block diagram of the compound optical circuit switch of FIG.4 illustrating simple and compound connections.

FIG. 6 is a block diagram of another compound optical circuit switch.

FIG. 7 is a block diagram of another compound optical circuit switch.

FIG. 8 is a block diagram of another compound optical circuit switch.

FIG. 9 is a block diagram of another compound optical circuit switch.

FIG. 10 is a flow chart of a process for optimizing a connection througha compound optical circuit switch.

FIG. 11 is a flow chart of another process for optimizing a connectionthrough a compound optical circuit switch.

FIG. 12 is a flow chart of a process for making a connection through acompound optical circuit switch.

FIG. 13 is a flow chart of another process for making a connectionthrough a compound optical circuits switch.

FIG. 14 is a flow chart of a process for rebuilding connections througha compound optical circuit switch.

Throughout this description, elements appearing in figures are assignedthree-digit reference designators, where the most significant digit isthe figure number where the element is introduced and the two leastsignificant digits are specific to the element. An element that is notdescribed in conjunction with a figure may be presumed to have the samecharacteristics and function as a previously-described element havingthe same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

Referring now to FIG. 1, an exemplary optical circuit switch 100 may beconfigured to connect a group of n inputs (where n is an integer greaterthan 1), labeled In 1 to In n, to a group of n outputs, labeled Out 1 toOut n. More specifically, the optical circuit switch 100 may selectivelyconnect up to n pairs of inputs and outputs.

Each of the inputs In 1 to In n may include a connector (of which onlythe connector 110-1 is identified) to receive an input optical signalfrom a optical fiber cable (not shown) external to the optical circuitswitch. Each connector may be coupled by a respective optical fiber (ofwhich only optical fiber 112-1 is identified) to a respective tapcoupler (of which only tap coupler 114-1 is identified). Each tapcoupler may extract an input sample portion, for example 1% to 10%, ofthe input optical signal from the respective optical fiber. Each inputsample portion may be directed to an input optical monitoring module170. The remainder of the input optical signals, other than the inputsample portions, may be conveyed by respective optical fibers torespective collimator lenses (of which only collimator lens 118-1 isidentified). Each collimator lens may convert the input optical signalfrom the respective optical fiber into a collimated input optical beam(of which only input optical beam 120-1 is identified) in free space.Free space optical beams are shown in FIG. 1 as dashed lines.

Each input optical beam, such as input optical beam 120-1, may bedirected onto a first mirror array 130. The first mirror array 130 mayinclude n mirrors with a one-to-one correspondence between input opticalbeams and mirrors, such that each input optical beam is directed onto arespective mirror. To improve the manufacturing yield of the firstmirror array, the first mirror array 130 may include more than nmirrors, in which case the n input optical beams may be directed to asubset of n mirrors that are known to be fully functional. Since each ofthe n input optical beams originates from a specific port and isdirected onto a specific mirror, each port may be described as “uniquelyassociated” with a corresponding mirror. In this patent, “uniquelyassociated” means a one-to-one correspondence. To take advantage of theavailable fully functional mirrors, the associations between ports andmirrors may be different in different optical circuit switches

Each mirror on the first mirror array 130 may reflect the respectiveinput optical beam to a selected mirror of a second mirror array 140.The mirrors of the second mirror array 140 may reflect the incident beamto form a respective output optical beam (of which only output opticalbeam 160-1 is identified). Each output optical beam may be directed to acorresponding focusing lens (of which only focusing lens 158-1 isidentified). Each focusing lens may focus the respective output opticalbeam into an output optical signal in a respective optical fiber. Eachoutput optical signal may be conveyed to a respective output tap coupler(of which only output tap coupler 154-1 is identified). Each output tapcoupler may direct a sample portion (for example 1% to 10%) of therespective output optical signal to an output optical monitoring module180. The remainder of each output optical signal, other than therespective sample portion, may be conveyed to a respective outputconnector (of which only connector 150-1 is identified).

The input optical monitoring module 170 and the output opticalmonitoring module 180 may be a common module. The input opticalmonitoring module 170 and the output optical monitoring module 180 maymeasure the optical power in each of the input sample portions andoutput sample portions, respectively. Each of the input opticalmonitoring module 170 and the output optical monitoring module 180 mayinclude an optical power detector for each sample portion.Alternatively, each of the input optical monitoring module 170 and theoutput optical monitoring module 180 may time-multiplex a singledetector or an array of detectors such that each detector measures theoptical power of sequence of sample portions. For example, each of theinput optical monitoring module 170 and the output optical monitoringmodule 180 may use a scanning mirror to direct sample portions to asingle detector or an array of detectors as described in U.S. Pat. No.7,676,125.

Sample portions may be extracted from the input optical beams, such asinput optical beam 120-1, and/or the output optical beams, such asoutput optical beam 160-1, using one or more free space sampling opticalelements. For example, sample portions may be extracted as described inU.S. Pat. No. 6,597,825 or U.S. Pat. No. 6,668,118. Input tap couplers,such as input tap coupler 114-1 and/or output tap couplers, such asoutput tap coupler 154-1, may not be present when free-space samplingoptical elements are used to extract sample portions.

The optical circuit switch 100 may include a controller 190. Thecontroller 190 may control the mirror elements in the first mirror array130 and the second mirror array 140 to make desired optical connectionsbetween the input In 1 to In n and the outputs Out 1 to Out n. Forexample, as shown in FIG. 1, input In 1 is connected to output Out 2,input In 2 is connected to output Out n, and input In n is connected tooutput Out 1. The controller 190 will be discussed in greater detailsubsequently.

The detail view 115 shows a simplified schematic diagram of a mirrorelement from either the first mirror array 130 or the second mirrorarray 140. A reflective mirror element 142 is supported by a pair oftorsion bars, of which only a first torsion bar 144 is visible. Thesecond torsion bar is located on the far side of the mirror element 142and axially aligned with the first torsion bar 144. The mirror element142 may rotate about the axis of the torsions bars, with the torsionbars providing a spring force tending to return the mirror element 142to a default position. The mirror element 142 may be coupled to a firstelectrode 146 and a second electrode 148. The mirror element 142 may berotated by electrostatic attraction between the mirror element andeither the first electrode 146 or the second electrode 148.

For example, applying a voltage between the first electrode 146 and themirror element 142 will create an attraction between the mirror elementand the first electrode, causing the mirror element to rotate in acounter-clockwise direction. The mirror will rotate until the returnforce of the torsion bars is equal to the force of the electrostaticattraction. The angular rotation of the mirror element 142 may beapproximately proportional to the square of the voltage between thefirst electrode 146 and the mirror element 142. Similarly, applying avoltage between the second electrode 148 and the mirror element 142 willcause the mirror to rotate in a clockwise direction. The first electrode146 and the second electrode may be “dedicated” to the mirror element142, which is to say the only function of the electrodes 146 and 148 isto rotate the mirror element 142 and the voltages applied to theelectrodes 146 and 148 have no effect on any mirror element other thanthe mirror element 142.

In the simplified example of FIG. 1, the mirror element 142 rotatesabout a single axis defined by the torsion bars 144. Either or both ofthe first mirror array 130 and the second mirror array 140 may includemirrors configured to independently rotate about two orthogonal axes. Inthis case, each mirror element may be coupled to a first pair ofelectrodes to cause clockwise and counter-clockwise rotation about afirst axis and a second pair of electrodes to cause clockwise andcounter-clockwise rotation about a second axis orthogonal to the firstaxis. The structure of a mirror array and the associated electrodes maybe substantially more complex than that shown in the simplifiedschematic detail view 115. For example, U.S. Pat. No. 6,628,041describes a MEMS mirror array having two-axis mirror motion and combactuators.

Referring now to FIG. 2, an environment 295 for the application of anoptical circuit switch 200 may include a network 290 and a networkcontroller 210. The optical circuit switch 200 may be the opticalcircuit switch 100 or may be a compound optical circuit switch includingmultiple copies of the optical circuit switch 100. When the opticalcircuit switch 200 is a compound optical circuit switch, the multiplecopies of the optical circuit switch 100 may be collocated ordistributed. The optical circuit switch 200 may be disposed within thenetwork 290 and may function to switch optical connections between othernodes (not shown) within the network 290. The network 290 may be, forexample, a wide area network, a local area network, a storage areanetwork, a private network within a data center or computer cluster, andmay be or include the Internet. While the connections switched by theoptical circuit switch 200 are optical, other connections within thenetwork 290 may be wired and/or wireless.

The network controller 210 may be a computing device that provides agraphic user interface or a command line interface for a networkoperator to enter connection commands (i.e. commands to make or breakone or more optical connections) for the optical circuit switch 200. Thenetwork controller 210 may be a computing device running networkmanagement software, in which case connection commands for the opticalcircuit switch 200 may be generated automatically by the networkcontroller 210.

A communications link 215 between the optical circuit switch 200 and thenetwork controller 210 may be in-band, which is to say thecommunications link 215 may be a path within the network 290. In thiscase, the optical circuit switch may have a wired, wireless, or opticalconnection to the network in addition to the optical connections beingswitched. The communications link 215 may be out-of-band, which is tosay the communications link 215 may be a dedicated connection or aconnection via a command network independent from the network 290. Aconfiguration in which the network controller 210 executes networkmanagement software to automatically provide connection commands to theoptical circuit switch 200 via an out-of-band communications link 215 isan example of what is commonly called a “software defined network”.

FIG. 3 is a high-level block diagram of the control and mirror driverportions of an optical circuit switch 300, which may be the opticalcircuit switch 100. The optical circuit switch 300 may include a switchcontroller 390, an input optical monitoring module 370, an outputoptical monitoring module 380, and a plurality of mirror driver circuits350. The optical circuit switch 300 may include one mirror drivercircuit 350 for each mirror in two mirror arrays if the individualmirror elements are rotatable about a single axis. The optical circuitswitch 300 may include two mirror driver circuits 350 for each mirror inthe mirror arrays if the individual mirror elements are rotatable abouttwo orthogonal axes. Each mirror driver circuit 350 may have, forexample, two selectable outputs to drive one or the other of a pair ofelectrodes, as described in pending patent application Ser. No.13/787,621.

The switch controller 390 may include a command interpreter 320 and aposition optimizer 330 which jointly maintain a connection state table340. The switch controller 390 may receive connection commands from anexternal source such as the network controller 210. The switchcontroller 390 may receive connection commands from some other source orin some other manner.

The command interpreter 320 may be responsive to a set of connectioncommands received by the switch controller 390. The set of connectioncommands may include, for example “Break a-b” and “Make a-b”. Thesecommands may respectively instruct the optical circuit switch 300 toeither break an existing connection between ports a and b (where a is aninteger input number and b is an integer output number), or to make anew connection between ports a and b. The set of connection commands mayinclude a mass connection command, which may list multiple connectionsto be made. The mass connection command may be used, for example, whenthe optical circuit switch is initially integrated into a network orwhen substantial reconfiguration of the network or data center isrequired.

The command interpreter 320 may include or have access to a port map322. As previously described, to allow the use of mirror arrays with asmall number of nonoperational mirror elements, the number of mirrorelements in each mirror array may be larger than the number of inputs oroutputs. Each input and output may be coupled to a known operationalmirror element in the respective mirror array. The port map 322 may be atable containing data relating each input to a mirror element in a firstmirror array, and data relating each output to a mirror element in asecond mirror array. The data in the port map 322 may be specific to theparticular first and second mirror arrays used in the optical circuitswitch 300.

There may be some performance variation from mirror element to mirrorelement and/or from mirror array to mirror array. In particular, theremay be some variation in the mirror element rotation angle versusapplied voltage characteristics within and between mirror arrays. Thecommand interpreter 320 may include or have access to a mirrorcalibration table 324 which contains data describing the performance ofeach mirror element. For example, the mirror calibration table 324 maystore the rotation angle versus voltage characteristic of each mirrorelement. The mirror calibration table 324 may store, for all possiblepairs of input and output mirror elements, a set of voltages that, whenapplied to the appropriate electrodes, will cause the mirror elements torotate to make the desired connection. The data in the mirrorcalibration table 324 may be specific to the particular mirror arraysused in the optical circuit switch 300. The data in the mirrorcalibration table 324 may be derived, for example, from the results oftests performed on the particular mirror arrays used in the opticalcircuit switch 300.

The data stored in the mirror calibration table 324 may indicate nominalvoltages required to initially make desired connections through theoptical circuit switch 300. However, once voltages are applied toelectrodes associated with a pair of input and output mirror elements toinitially make a connection, the positions of the mirror elements maydrift over time. The result of mirror element drift may be failure ordegradation (e.g. increased insertion loss) of the connection. Themirror arrays used in the optical circuit switch 300 may be fabricatedby chemical micromachining of a silicon substrate. For example, eachmirror element may consist of a reflective coating on a silicon slabthat is connected to the silicon substrate by narrow silicon elementsthat function as torsion bars. Each silicon mirror slab may be free torotate about the axis or axes defined by the torsion bars. Mirrorelement drift may be due to mechanical stress relief of the torsion barsover time. Further, all or portions of the silicon surfaces of themirror array may be coated with SiO2 or some other dielectric. Electriccharge trapped at defects in the insulators layers may contribute tomirror element drift over time. Other causes may also contribute tomirror element drift.

The position optimizer 330 may receive data from the input opticalmonitoring module 370 and the output optical monitoring module 380indicating the power levels at the inputs and the outputs, respectively.The position optimizer 330 may determine the insertion loss of eachactive optical connection (i.e. each optical connection where light ispresent) from the respective input and output power levels. The positionoptimizer 330 may periodically adjust the positions of some or all ofthe mirror elements to minimize the insertion loss of each opticalconnection. For example, to optimize a connection, the positionoptimizer 330 may make incremental changes in the position of one of themirror elements used in the connection and observe the resulting effecton insertion loss. The optimum position of the mirror element may thenbe found using a hill climbing algorithm or a similar algorithm. Theposition of each mirror element may be optimized periodically. The timeinterval between successive optimizations of each mirror element may beshort (on the order of seconds) compared to the time constant of themirror element drift (on the order of hours). Periodic optimization ofthe position of each mirror element may automatically compensate formirror element drift.

The command interpreter 320 and the position optimizer 330 may jointlymaintain and share the connection state table 340. The connection statetable 340 may include data indicative of the state or status of eachport of the optical circuit switch 300. Data included in the connectionstate table 340 for each port may include a first flag indicating if therespective port is available or committed to a connection, and a secondflag indicating if the connection has actually been made. The connectionstate table 340 may include, for inputs, a third flag indicating islight is present at the respective input. For each port that iscommitted to a connection, the connection state table 340 may alsoinclude the identity of the port at the other end of the connection, themirror element associated with the port, the voltages presently appliedto the electrodes associated with the mirror element, and temporal datasuch as when the connection was first made and when the position of themirror element was most recently optimized.

Referring now to FIG. 4, an exemplary compound optical circuit switch400 may be capable of making a unidirectional connection between any of640 input ports and any of 640 output ports. The 640 input ports and 640output ports may be grouped in pairs to provide 640 full-duplex ports,as shown in FIG. 4. In this context, a full-duplex port is an inputpaired with an output to accommodate bidirectional communications. Thecompound optical circuit switch 400 may be capable of making abidirectional connection between any one of 640 full-duplex ports to anyother of the 640 full duplex ports. In this context, a full-duplex portis an input paired with an output to accommodate bidirectionalcommunications. The compound optical circuit switch 400 may be fullynon-blocking, which is to say a bidirectional connection between any twoselected full-duplex ports can be made regardless of any possiblecombination of existing connections between other full-duplex ports.

The compound optical circuit switch 400 may include six optical circuitswitches 410-1 to 410-4 and 420-1 to 420-2, each of which may be theoptical circuit switch 100. Each of the optical circuit switches 410-1to 410-4 and 420-1 to 420-2 may be configured to connect a group of 320inputs to a group of 320 outputs. More specifically, each of the opticalcircuit switches 410-1 to 410-4 and 420-1 to 420-2 may selectively makeup to 320 connections, where each connection conveys an optical signalfrom an input to an output. The choice of 320 input and outputs peroptical circuit switch is exemplary, and the optical circuit switches410-1 to 410-4 and 420-1 to 420-2 may have more or fewer inputs andoutputs.

The six 320×320 optical circuit switches may be arranged as four1^(st)-tier switches 410-1, 410-2, 410-3, and 410-4 and two 2^(nd)-tierswitches 420-1, 420-2. The 640 full-duplex ports of the compound opticalcircuits switch 400 may be connected to respective input/output pairs ofthe 1^(st)-tier optical circuit switches 410-1 to 410-4. Specifically160 inputs and 160 outputs of each 1^(st)-tier optical circuit switchmay be connected respectively to 160 full-duplex ports. For example, the160 inputs and 160 outputs of optical circuit switch 410-1 may beconnected to full-duplex ports numbered from 1 to 160. Similarly,optical circuit switch 410-2 may be connected to full-duplex ports 161to 320, optical circuit switch 410-3 may be connected to full-duplexports 321 to 480, and optical circuit switch 410-4 may be connected tofull-duplex ports 481 to 640. The full-duplex ports may be numbered insome other manner.

The other inputs and outputs of the 1^(st)-tier optical circuit switches410-1 to 410-4 may be connected to the 2^(nd)-tier optical circuitswitches 420-1 and 420-2. Specifically 80 outputs of each of the four1^(st)-tier optical circuit switches 410-1 to 410-4 may be connected torespective inputs of each of the 2^(nd)-tier optical circuit switches420-1, 420-2. 80 inputs of each of the four 1^(st)-tier optical circuitswitches 410-1 to 410-4 may be connected to respective outputs of eachof the 2^(nd)-tier optical circuit switches 420-1, 420-2.

Referring now to FIG. 5, connections between full-duplex ports providedon the same 1^(st)-tier optical circuit switch may be made within the1^(st)-tier optical circuit switch. For example, a connection betweentwo of the full duplex ports 161 to 320 can be made within the1^(st)-tier optical circuit switch 410-2, as illustrated by the arrow502. Such connections will be referred to herein as “simpleconnections”. A special case of a simple connection is when the inputand output of a full duplex port are connected to each other to providean optical loop-back. A loop-back connection may be useful duringdiagnostic testing of a network.

Connections between full-duplex ports provided by different 1^(st)-tieroptical circuit switches 410-1 to 410-4 may be made via one of thesecond tier optical circuit switches 420-1 and 420-2, as illustrated bythe optical path 504. Such connections will be referred to herein as“compound connections”, where “compound” has its normal meaning of“composed of the union of several elements”. Each compound connectionincludes connections through two different 1^(st)-tier optical circuitswitches and a connection through a 2^(nd)-tier optical circuit switch.These connections will be referred to, in the direction of opticalsignal flow, an “input 1^(st) tier connection” 506, a “2^(nd)-tierconnection” 508, and an “output 1^(st)-tier connection” 510. Theswitches involved in a compound connection will be referred to, in thedirection of optical signal flow, the input 1^(st)-tier switch, the2^(nd)-tier switch, and the output 1^(st)-tier switch.

Referring back to FIG. 4, the compound optical circuit switch 400 mayinclude a compound connection controller 430 having primaryresponsibility for routing compound connections through compound opticalcircuit switch. The compound connection controller 430 may receiveglobal connection commands from a network controller such as the networkcontroller 210. In this context, a “global connection command” is acommand directed to the compound optical circuit switch 400 as a whole,as opposed to a “local connection command” directed to one n×n opticalcircuit switch 410-1 to 410-4, 420-1, 420-2 within the compound opticalcircuit switch 400. Each global connection command may require one ormore connections through the compound optical circuit switch 400 to bemade and/or broken. When a received global connection command calls fora simple connection to be made or broken, the compound connectioncontroller 430 may relay the global connection command as a localconnection command to the appropriate 1^(st) tier optical circuit switch410-1 to 410-4. When a received global connection command calls for acompound connection to be made or broken, the compound connectioncontroller 430 may select a path through the compound optical circuitswitch 400 and send individual local connection commands to theappropriate 1^(st)-tier optical circuit switches 410-1 to 410-4 and theselected and 2^(nd)-tier optical circuit switch 420-1 or 420-2.

Referring now to FIG. 6, a generalized compound optical circuit switch600 may be capable of making a unidirectional connection between any ofkn input ports and any of kn output ports, where n is a number of inputsand outputs provided by an n×n optical circuit switch and k is aninteger greater than one. The kn input ports and kn output ports may begrouped in pairs to provide kn full-duplex ports. The compound opticalcircuit switch 600 may be capable of making a bidirectional connectionbetween any one of the kn full-duplex ports to any other of the kn fullduplex ports. The compound optical circuit switch 600 may be fullynon-blocking, which is to say a bidirectional connection between any twoselected full-duplex ports can be made regardless of any possiblecombination of existing connections between other full-duplex ports.

The compound optical circuit switch 600 may include 2k n×n 1^(st)-tieroptical circuit switches. Each of the 2k n×n 1^(st)-tier switches may beconfigured to connect a group of n inputs to a group of n outputs. Eachof the 2k n×n 1^(st)-tier switches may selectively make up to nconnections, where each connection conveys an optical signal from aninput to an output. The kn full-duplex ports of the compound opticalcircuit switch 600 may be connected to respective input/output pairs ofthe 1^(st)-tier optical circuit switches 610-1 to 610-2k. Specificallyn/2 inputs and n/2 outputs of each 1^(st)-tier optical circuit switchmay be paired to provide n/2 full-duplex ports.

The compound optical circuit switch 600 may include j m×m 2^(nd)-tieroptical circuit switches 620-1 to 620-j. Each of the j m×m 2^(nd)-tierswitches may be configured to connect a group of m inputs to a group ofm outputs. Each of the j m×m 2^(nd)-tier switches may selectively makeup to m connections, where each connection conveys an optical signalfrom an input to an output.

The other inputs and outputs of the 1^(st)-tier optical circuit switches610-1 to 610-2k may be connected to the 2^(nd)-tier optical circuitswitches 620-1 and 620-j. Specifically n/2j outputs of each of the 2k1^(st)-tier optical circuit switches 610-1 to 610-2k may be connected torespective inputs of each of the 2^(nd)-tier optical circuit switches620-1, 620-j. n/2j inputs of each of the 2k 1^(st)-tier optical circuitswitches 610-1 to 610-2k may be connected to respective outputs of eachof the 2^(nd)-tier optical circuit switches 620-1, 620-j. In order foreach input or output of a 1^(st)-tier switch to be connected to arespective output or input of a 2^(nd)-tier switch, the relationshipkn=jm may be satisfied. The compound optical circuit switch 400 of FIG.4 is an instantiation of the compound optical circuit switch 600 withn=m=320 and k=j=2. In other embodiments of the compound optical circuitswitch 600, j, k, m, and n may have other values.

The compound optical circuit switch 600 may include a compoundconnection controller 630 having primary responsibility for routingcompound connections through compound optical circuit switch. Thecompound connection controller 630 may receive global connectioncommands from a network controller such as the network controller 210.Each global connection command may require one or more connectionsthrough the compound optical circuit switch 600 to be made and/orbroken. When a received global connection command calls for a simpleconnection to be made or broken, the compound connection controller 630may simply relay the global connection command as a local connectioncommand to the appropriate 1^(st) tier optical circuit switch 610-1 to610-2k. When a received global connection command calls for a compoundconnection to be made or broken, the compound connection controller 630may select a path through the compound optical circuit switch 600 andsend individual local connection commands to the appropriate 1^(st)-tieroptical circuit switches 610-1 to 610-2k and the selected 2^(nd)-tieroptical circuit switch 620-1 to 620-j.

Briefly reviewing the description of FIG. 1, an optical circuit switch100 may include tap couplers (of which only tap coupler 114-1 isidentified) to direct a sample portion of each input signal to an inputoptical monitoring module 170 that measures an optical power level ofeach input. The optical circuit switch 100 may also include tap couplers(of which only tap coupler 154-1 is identified) to direct a sampleportion of each output signal to an output optical monitoring module 180that measures an optical power level of each output. A controller 190may control the position of mirrors within mirror arrays 130, 140 tooptimize each connection through the optical circuit switch 100 based onthe respective input and output power measurements.

However, in an compound optical circuit switch such as the compoundoptical circuit switch 600, every input to a 2^(nd)-tier switch 620-1 to620-j comes from an output of one of the 1^(st)-tier switches 610-1 to610-2k. Thus, ignoring possible losses in the fiber optic connectionsbetween the 1^(st)-tier and 2^(nd)-tier switches, the input power levelsat the inputs of the 2^(nd)-tier switches will be the same as the outputpower levels at the outputs of the 1^(st)-tier switches. Thus it is notnecessary for a compound optical circuit switch to include both outputpower monitoring in the 1^(st)-tier switches and input power monitoringin the 2^(nd)-tier switches. Similarly, every output from a 2^(nd)-tierswitch 620-1 to 620-j goes to an input of one of the 1^(st)-tierswitches 610-1 to 610-2k. Thus it is not necessary for a compoundoptical circuit switch to include both output power monitoring in the2^(nd)-tier switches and input power monitoring in the 1^(st)-tierswitches. Power monitoring in the 1^(st-)tier switches alone willperform the functions for both the 2^(nd)-tier and 1^(st)-tier switchesbecause all connections, both simple and compound, must pass through atleast one of the 1^(st)-tier switches.

Referring now to FIG. 7, another compound optical circuit switch 700 maybe capable of making a bidirectional connection between any one of knfull-duplex ports to any other of the kn full duplex ports, where n is anumber of inputs and outputs provided by an n×n optical circuit switchand k is an integer greater than zero. The compound optical circuitswitch 700 may include 2k first-tier switches 710-1 to 710-2k and jsecond-tier switches 720-1 to 720-j The 1^(st)-tier and 2^(nd)-tierswitches may be interconnected as described with respect to FIG. 6. Thecompound optical circuit switch 700 may include a compound connectioncontroller 730 having primary responsibility for routing compoundconnections through compound optical circuit switch as previouslydescribed.

Each 1^(st)-tier switch 710-1 to 710-2k may include tap couplers on eachinput and output and respective input optical monitoring modules 770-1to 770-2k and output optical monitoring modules 780-1 to 780-2k. Each2^(nd)-tier switch 720-1 to 720-j may not include taps couplers oroptical monitoring modules. Instead, the respective switch controllers725-1 to 725-j of the 2^(nd)-tier switches may receive power measurementdata 718-1 to 718-2k from the respective switch controllers 715-1 to715-2k of the 1^(st) tier switches. Power measurement data 718-1 to718-2k may be communicated from the 1^(st)-tier switches to the2^(nd)-tier switches via multiple dedicated connections (as shown) orvia one or more shared bus or network. For example, the 1^(st)-tierswitches 710-1 to 710-2k may transmit power measurement data in rotationover a shared bus or network. Each 1^(st)-tier switch may transmit powermeasurement data for all inputs and outputs, or only those inputs andoutputs connected to one of the 2^(nd)-tier switches 720-1 to 720-j.Each 2^(nd)-tier switch (or the switch controllers 725-1 to 725-j or thecompound connection controller 730) may have knowledge of theinterconnections between the 1^(st)-tier switches and each 2^(nd)-tierswitch but power measurement data may only be captured for only the1^(st)-tier inputs and outputs to which it is actually connected.

Referring now to FIG. 8, another compound optical circuit switch 800 maybe capable of making a bidirectional connection between any one of knfull-duplex ports to any other of the kn full duplex ports, where n is anumber of inputs and outputs provided by an n×n optical circuit switchand k is an integer greater than zero. The compound optical circuitswitch 800 may include 2k first-tier switches 810-1 to 810-2k and jsecond-tier switches 820-1 to 820-k. The 1^(st)-tier and 2^(nd)-tierswitches may be interconnected as described with respect to FIG. 6. Thecompound optical circuit switch 800 may include a compound connectioncontroller 830 having primary responsibility for routing compoundconnections through compound optical circuit switch as previouslydescribed.

Each 1^(st)-tier switch 810-1 to 810-2k may include tap couplers on eachinput and output and respective input optical monitoring modules 870-1to 870-2k and output optical monitoring modules 880-1 to 880-2k. Each2^(nd)-tier switch 820-1 to 820-j may not include taps couplers oroptical monitoring modules. Instead, the respective switch controllers815-1 to 815-2k of the 1^(st)-tier switches may provide powermeasurement data 818-1 to 818-2k to a master controller 840. The mastercontroller 840 may be a separate computing device, or may be integratedwith the compound connection controller 830 or one of the switchcontrollers 815-1 to 815-2k or 825-1 to 825-j. Power measurement data718-1 to 718-2k may be communicated from the 1^(st)-tier switches to themaster controller 840 via multiple dedicated connections (as shown) orvia one or more shared bus or network. The master controller may beknowledgeable of the interconnections between the 1^(st)-tier and2^(nd)-tier switches. The master controller 840 may distribute theappropriate power measurement data 835-1 to 835-j to each 2^(nd)-tierswitch 820-1 to 820-j. The master controller 840 may also coordinateoptimization of compound connections between the 1^(st)-tier and2^(nd)-tier switches.

Referring now to FIG. 9, another compound optical circuit switch 900 maybe capable of making a bidirectional connection between any one of 512full-duplex ports to any other of the 512 full duplex ports. Thecompound optical circuit switch 900, like the compound optical circuitswitch 400 of FIG. 4, may include four first-tier switches 910-1 to910-4, each of which can make half-duplex connections between 320 inputand output ports. Unlike the compound optical circuit switch 400, 128input and output ports of each 1^(st)-tier switch 910-1 to 910-4 areused to provide a total of 512 full duplex ports of the compound opticalcircuit switch. 192 input and output ports of each 1^(st)-tier switch910-1 to 910-4 are connected to three second-tier switches 920-1 to920-3. The addition of a third 2^(nd)-tier switch provides redundancysuch that all possible full-duplex connections through the compoundoptical circuit switch 900 can be made even in the event of a completefailure of one of the 2^(nd) tier switches 920-1 to 920-3. Redundancy isprovided at the expense of a reduction in the number of full duplexports (512 as opposed 640 for the compound optical circuit switch 400)and the addition of the third 2^(nd)-tier switch. Redundancy may beprovided in higher port count compound optical circuits switches byincorporating at least one redundant 2^(nd)-tier switch connected toinput and output ports of each 1^(st)-tier switch

Description of Processes

FIG. 10 is a flow chart of a process 1000 for optimizing a connectionthrough a compound optical circuit switch such as the compound opticalcircuit switches 400, 600, 700, and 800. The process 1000 may start at1011 and end at 1090 after a single unidirectional connection has beenoptimized. Although the process 1000 is not inherently cyclic, theprocess 1000 may be repeated periodically, as indicated by the dashedarrow 1095, to re-optimize the connection to correct for factors such asmirror element drift and/or temperature changes.

Multiple instantiations of the process 1000 may proceed sequentiallyand/or concurrently to optimize a large number of connections throughthe compound optical circuit switch. Note that two instantiations of theprocess 1000 are required to optimize both directions of a full-duplexsimple connection. For example, the compound optical circuit switch 400of FIG. 4 can provide full-duplex connections between 640 full-duplexports, which is equivalent to 640 unidirectional connections betweenindividual inputs and outputs. The compound optical circuit switch 400may run up to 640 instantiations of the process 1000 to optimize allconnections. The compound optical circuit switches 600, 700, and 800 mayrun up to kn instantiations of the process 1000.

At 1020 a determination may be made whether or not the connection to beoptimized is a simple connection (see, e.g., simple connection 520 inFIG. 5). When the connection to be optimized is a simple connection(“yes” at 1020), the connection is made through a single 1^(st)-tierswitch. In this case, the mirror positions in the 1^(st)-tier switch maybe optimized at 1030 using a local search optimization technique. Forexample, a hill climbing algorithm may be used to optimize the mirrorposition. A mirror position may be incrementally changed and adetermination may be made if the position change improved or degradedthe insertion loss of the connection through the switch. Successiveincremental changes may be made until an optimum mirror position isdetermined. Sequential hill-climbing algorithms may be performed foreach of two rotation axes and two mirror elements for each connection.After the positions of both mirror elements on both axes have beenoptimized, the process 1000 may end at 1090.

When a determination is made at 1020 that the connection is not a simpleconnection (“no” at 1020), the connection must be, by default, acompound connection through two 1^(st)-tier switches and a 2^(nd)-tierswitch. In this case, the mirror positions within the three switches maybe optimized sequentially at 1040, 1050, and 1060 using hill climbing orother optimization algorithms as previously described. Sequentialoptimization is necessary to avoid the ambiguity that would inherentlyoccur if hill climbing algorithms were run simultaneously on multiplemirror elements along the same connection. The order in which the mirrorpositions in the three switches are optimized is unimportant, so long asthe mirror positions in only one switch are being optimized at any giventime. After the mirror positions within the three switches have beenoptimized, the process 1000 may end at 1090.

The process 1000 may be supervised by a master controller such as themaster controller 840 in the compound optical circuit switch 800. Themaster controller may control the sequence 1040, 1050, 1060 in which1^(st)-tier and 2^(nd)-tier switch optimize connections. For example,for each compound connection, the master controller may authorize afirst switch to optimize mirror positions at 1040. After receivingconfirmation from the first switch that its optimization has beencompleted, the master controller may authorize the second switch tooptimize mirror positions at 1050 and, subsequently, the third switch tooptimize mirror positions at 1060. The master controller may alsodetermine how often the process 1000 should be repeated (dashed arrow1095).

FIG. 11 is a flow chart of another process 1100 for optimizing aconnection through a compound optical circuit switch that does notinclude a master controller, such as the compound optical circuitswitches 400, 600, and 700. The process 1100 may start at 1105 when afirst connection is made through the compound optical circuit switch andmay continue perpetually so long as the compound optical circuit switchis in service. The process 1100 is inherently cyclic, and will repeatperiodically to re-optimize the connection to correct for factors suchas mirror element drift and/or temperature changes.

Multiple instantiations of the process 1100 may proceed sequentiallyand/or concurrently to optimize a large number of connections throughthe compound optical circuit switch. Note that two instantiations of theprocess 1100 are required to optimize both directions of a full-duplexsimple connection. For example, the compound optical circuit switch 400of FIG. 4 can provide full-duplex connections between 640 full-duplexports, which is equivalent to 640 unidirectional connections betweenindividual inputs and outputs. The compound optical circuit switch 400may run up to 640 instantiations of the process 1100 to optimize allconnections. The compound optical circuit switches 600 and 700 may runup to kn instantiations of the process 1100.

As shown in FIG. 11, the process 1100 is controlled by the input1^(st)-tier switch for each connection. An alternate version of theprocess 1100 may be controlled by the output 1^(st)-tier switch of eachconnection.

At 1110, the input 1^(st)-tier switch may determine whether or not it istime to start a next optimization cycle. This determination may bebased, for example, by a hardware or software timer that initiates anoptimization cycle at periodic intervals. It the determination is madethat it is not yet time to begin a new cycle (“no” at 1110) the process1100 may idle at 1110.

When a determination is made at 1110 to initiate a new optimizationcycle, the positions of mirrors within the input 1^(st)-tier switch maybe optimized at 1120 using a local search optimization technique. Forexample, a hill climbing algorithm may be used to optimize the mirrorposition. A mirror position may be incrementally changed and adetermination may be made if the position change improved or degradedthe insertion loss of the connection through the switch. Successiveincremental changes may be made until an optimum mirror position isdetermined. Sequential hill-climbing algorithms may be performed foreach of two rotation axes and two mirror elements for each connection.

After the positions of both mirror elements on both axes have beenoptimized at 1120, a determination may be made at 1130 whether or notthe connection is a simple connection. When the connection is a simpleconnection (“yes” at 1130), the connection is completed within the input1^(st)-tier switch (which is inherently also the output 1^(st)-tierswitch). Thus the optimization performed at 1120 optimizes the entiresimple connection. In this case, the process 1100 may return to 1110 toawait the start of the next cycle.

When a determination is made at 1130 that the connection is not a simpleconnection (“no” at 1130), the connection must be, by default, acompound connection through two 1^(st)-tier switches and a 2^(nd)-tierswitch. In this case, at 1140, the input 1^(st)-tier switch may pass atoken to the appropriate 2^(nd)-tier switch. The token may be passed,for example, using the same bus, network, or other communications pathused to pass power measurements from the 1^(st)-tier switches to the2^(nd)-tier switches.

Upon receipt of the token, the 2^(nd)-tier switch may optimize thepositions of the appropriate mirrors at 1150. When complete, the2^(nd)-tier switch may pass the token to the appropriate output 1^(st)tier switch at 1160. Upon receipt of the token, the output 1^(st)-tierswitch may optimize the positions of the appropriate mirrors at 1170 tocomplete the optimization of the entire compound connection. The process1105 may then return to 1110 to await the next optimization cycle.

FIG. 12 is a flow chart of a process 1200 for making a full-duplexconnection through a compound optical circuit switch, such as thecompound optical circuit switches 400, 600, 700, and 800. The process1200 may start at 1205 when a command to make a connection between todesignated ports is received by the compound optical circuit switch. Thecommand may be received, for example, from a network controller such asthe network controller 210. The process 1200 may end at 1295 after asingle full-duplex (bidirectional) connection has been made. The process1200 may be repeated each time a new connection is made through thecompound optical circuit switch.

At 1210, a determination may be made whether or not the two ports to beconnected are available, which is to say the two ports to be connectedare not already used in a prior connection. If one or both of the twoports is used in an existing connection and thus not available (“no” at1210), an error message may be issued to the network controller at 1220and the process 1200 may end at 1295. If a determination is made thatthe two ports are available (“yes” at 1210), the process 1200 mayproceed to 1220. The actions at 1210 and 1215 are optional and in somecompound optical circuit switches, the process 1200 may proceedimmediate to 1220 from the start at 1205.

At 1220, a determination may be made whether or not the connection to beoptimized is a simple connection. When the connection to be optimized isa simple connection (“yes” at 1220), the connection can be made througha single 1^(st)-tier switch. In this case, the connection may be madethrough the appropriate 1^(st)-tier switch at 1225 and the process 1200may end at 1295.

When a determination is made at 1220 that the connection is not a simpleconnection (“no” at 1220), the connection must be, by default, acompound connection through two 1^(st)-tier switches and a 2^(nd)-tierswitch. In this case, connection paths for each direction of thefull-duplex connection may be selected from among the available paths at1230. For example, when the first connection made though the compoundoptical circuit switch 400 is a compound connection, 80 different pathswill be available from the input 1st-tier switch to either of the2^(nd)-tier switches, and 80 different paths will be available from theselected 2^(nd)-tier switch to the output 1^(st)-tier switch, for atotal of 2×80×80=12,800 different possible routings for the connection.At the other extreme, after 639 connections have been made, only asingle connection path will be available for the 640^(th) connection.The paths between the 1^(st)-tier and 2^(nd)-tier switches may beselected at 1250 sequentially, randomly, or in some other manner. Thebidirectional paths of a full-duplex connection may commonly, but notnecessarily be routed through the same 2^(nd)-tier switch.

Particular connection paths for each direction of the full-duplexconnection may be selected from among the available paths at 1230randomly, or in a predetermined order, or based on one or more selectioncriteria. For example, a preferred 2^(nd)-tier switch may be specifiedin the command received at 1205. The various 1^(st)-tier and 2^(nd)-tierswitches making up a compound optical circuit switch are not necessarilyco-located. The 1^(st)-tier and 2^(nd)-tier switches of a compoundoptical circuit switch may be physically distributed between multipleequipment racks, multiple buildings, and even multiple cities. When the1^(st)-tier and 2^(nd)-tier switches of a compound optical circuitswitch are physically distributed, a selection criteria applied at 1230may be to minimize the physical distance of the paths. Minimizing thephysical distance of the paths may, for example, lower the probabilityof the connection being lost due to a cable failure. Different physicaland logical security levels may be imposed on different 1^(st)-tier and2^(nd)-tier switches making up a compound optical circuit switch. Inthis case, a selection criteria applied at 1230 may be to ensure aparticular security level as defined in the command received at 1205.Other criteria may be used at 1230 to select particular paths from aplurality of available paths. When multiple selection criteria areapplied to a full duplex connection, priorities or weights may beassigned to the selection criteria.

After the connections paths have been selected at 1230, the input 1^(st)tier switch may be instructed to make the input 1^(st)-tier connectionsat 1260. Making a connection in an optical circuit switch may require anextended period of time (for example tens or hundreds of milliseconds)since the mirror elements may be moved gradually to avoid mechanicalovershoot, ringing, and possible damage. Making the input 1^(st)-tierconnections at 1260 allows the input 1^(st)-tier connections beoptimized at 1265 (assuming light is present at the inputs to theconnections) without having to wait for the movement of the mirrors inthe 2^(nd)-tier switch and the output 1^(st)-tier switch. The2^(nd)-tier connections may be made at 1270, concurrent with theoptimization of the input 1^(st)-tier connection at 1265. The2^(nd)-tier connections may be optimized at 1275 after the optimizationof the input 1^(st)-tier connections at 1265 is complete. The output1^(st)-tier connections may be made last at 1280, which may beconcurrent with the optimization of the input 1^(st)-tier connection at1265 or optimization of the 2^(nd)-tier connection at 1275. The output1^(st)-tier connections may be optimized at 1285 after the optimizationof the 2^(nd)-tier connections at 1275 is complete. The process 1200 maythen end at 1295.

Portions of the process 1200 may be performed and/or supervised by acompound connection controller such as the compound connectioncontrollers 430, 630, 730 and 830. The compound connection controllermay receive connection commands at 1205, determine if the ports areavailable at 1200 and issue error messages at 1205. When a simpleconnection is made, the compound connection controller may relay theconnection command to the appropriate 1^(st)-tier switch to make theconnection at 1225. When a compound connection is made, the compoundconnection controller may select the connection path at 1230 and providelocal commands to the appropriate 1^(st)-tier and 2^(nd)-tier switchesto make the connection at 1260-1285.

Optical signals routed through optical circuit switch will sufferinsertion loss due to small, but cumulative, losses at the movablemirrors, collimating and focusing lenses, optical fibers, tap couplers,and other components of the optical circuit switch. Further, minorvariations in these components may result in variations in the expectedinsertion loss (i.e. the insertion loss after mirror positions areoptimized) of different connections through an optical circuit switch.The variations in insertion loss can be exaggerated in a compoundoptical circuit switch since each compound connection consists of threeoptical circuit switch connections in series. Excessive cumulativeinsertion loss over an optical communications path may result in lowsignal-to-noise ratio of the received optical signal which may, in turn,cause increased bit error rate. Network controllers, such as the networkcontroller 210, may strive to limit the cumulative insertion loss ineach optical communications path. To this end, a network controller mayestablish a loss budget for each communication path, which may includean insertion loss target for some or all connections through opticalcircuit switched.

FIG. 13 is a flow chart of another process 1300 for making a full-duplexconnection through a compound optical circuit switch, such as thecompound optical circuit switches 400, 600, 700, and 800. The process1300 is similar to the process 1200, except that the process 1300considers expected insertion loss when making connections and, inparticular, when selecting paths for compound connections. To this end,an expected loss for every possible connection through each of the1^(st)-tier and 2^(nd)-tier switches may be stored in an expected losstable 1335. The data in the expected loss table may be derived, forexample, from manufacturer's test data on each of the optical circuitswitches.

The process 1300 may start at 1305 when a command to make a connectionis received by the compound optical circuit switch. The actions 1310 to1325 and 1360 to 1385 in the process 1300 are essentially identical tothe counterpart actions in the process 1200. Complete descriptions ofsimilar actions will not be repeated. The process 1300 may end at 1395after a single full-duplex (bidirectional) connection has been made.

The command received at 1315 may be provided, for example, by a networkcontroller such as the network controller 210. The command may identifya pair of full-duplex ports to be connected. The command may optionallyinclude data indicating a respective insertion loss target for theconnection to be made. For example, the command received at 1305 mayspecify a maximum acceptable insertion loss for the connection. Thecommand received at 1305 may include a flag or other data indicating theconnection should be made with the minimum possible insertion loss. Thecommand may include a flag or other data indicating that the connectioncan tolerate relatively high insertion loss. The command received at1305 may include data specifying an insertion loss target in some othermanner.

When a determination is made at 1320 that a simple connection will bemade, there will be only a single possible optical path for theconnection within the appropriate one of the 1^(st)-tier switches. Ifthe command received at 1305 included a loss target for this connection,an expected loss of the connection may be retrieved from the expectedloss table 1335. The expected loss and the loss target may be comparedand a determination may be made at 1340 whether or not the expected losssatisfies the loss target. When the loss target is satisfied (“yes” at1340) the connection may be made at 1325 and the process 1300 may end at1395.

When a determination is made at 1340 that the loss target will not besatisfied (“no” at 1340), a message to that effect may be issued at1315, and the process 1300 may end at 1395. Alternatively, theconnection may be made anyway (for example when no alternativeconnection paths are available) at 1325, as indicated by the dashed line1345.

When a determination is made at 1320 that a compound connection isrequired (“no” at 1320), a connection path for the compound connectionmay be selected at 1330. In most circumstances, multiple (possiblyhundreds) of alternative connection paths may be available to make aparticular compound connection. Each connection path consists of aninput 1^(st)-tier connection, a 2^(nd)-tier connection, and an output1^(st)-tier connection. When multiple connection paths are available,the selection of a particular connection path at 1330 may be based onselection criteria as previously described. When multiple selectioncriteria are applied to a full duplex connection, priorities or weightsmay be assigned to the selection criteria.

One selection criteria that may be applied at 1330 is a loss target forthe connection. For example, if the loss target is to make theconnection with the minimum possible insertion loss, the expected losstable 1335 may be searched for the available path having the lowestinsertion loss. If the loss target is expressed as a maximum allowedloss level, the expected loss table 1335 may be searched for the firstavailable with an insertions loss lower than the target level. If theloss target indicates that the connection can tolerate high loss, theexpected loss table 1335 may be searched for the available path havingthe highest insertion loss (to preserve lower insertion lossconnections). The connection path may be selected at 1330 is some othermanner.

At expected loss of the selected connection path and the loss target maybe compared and a determination may be made at 1350 whether or not theexpected loss satisfies the loss target. When the loss target issatisfied (“yes” at 1350) the connection may be made at 1360-1385 aspreviously described. The process 1300 may then end at 1395.

When a determination is made at 1350 that the loss target will not besatisfied (“no” at 1350), a message to that effect may be issued at1315, and the process 1300 may end at 1395. Alternatively, theconnection may be made anyway (for example, when no alternativeconnection path is available) at 1360-1385, as indicated by the dashedline 1355.

In the absence of a loss target for a compound connection, or as analternative to the use of loss targets altogether, the connections pathsfor compound connections may be selected at 1330 to attempt to equalizethe insertion loss of all compound connections. To this end, theinsertion loss of each compound connection may be set, to the extentpossible, equal to the average expected insertion loss of a compoundconnection. At 1330, the connection path may be selected in accordancewith the formula

IL1in+IL2+IL1out≈2IL1av+IL2av  (1)

where IL1in, IL2, and IL1out are the insertion losses (from the expectedloss table 1335) of the selected connections through the input1^(st)-tier switch, the selected 2^(nd)-tier switch, and the output1^(st)-tier switch respectively; IL1av and IL2av are the averageinsertion losses of connections through 1^(st)-tier and 2^(nd)-tierswitches, respectively; and the symbol ≈ has its normal meaning of“almost equal to”.

Portions of the process 1300 may be performed and/or supervised by acompound connection controller such as the compound connectioncontrollers 430, 630, 730 and 830. The compound connection controllermay include the expected loss table 1335. The compound connectioncontroller may receive connection commands at 1315, determine if theports are available at 1310 and issue error messages at 1315. When asimple connection is made, the compound connection controller maydetermine if the connection will meet a lost target at 1340 and relaythe connection command to the appropriate 1^(st)-tier switch to make theconnection at 1325. When a compound connection is made, the compoundconnection controller may select the connection path at 1330, determineif a loss target is satisfied at 1350, and provide local commands to theappropriate 1^(st)-tier and 2^(nd)-tier switches to make the connectionat 1360-1385.

FIG. 14 is a flow chart of a process 1400 for rebuilding full-duplexcompound connections through a compound optical circuit switch, such asthe compound optical circuit switches 400, 600, 700, 800, and 900 in theevent of a failure of a 2nd-tier switch. The process 1400 may start at1410 when an alarm or other indication is received indicating failure ofa 2^(nd) tier switch within the compound optical circuit switch. Thealarm may be generated with the compound optical circuit switch or maybe surmised from externally generated information such as the disruptionof a plurality of connections made through the failed 2^(nd)-tierswitch. The process 1400 may end at 1490 when the connections broken bythe failure of the 2^(nd)-tier switch have be rebuilt to the extentpossible.

At 1420, the compound connections broken by the failure of the2^(nd)-tier switch may be identified and, optionally, prioritized. Forexample, a compound connection controller (e.g. 430, 630, 730, 830, 930)within the compound optical circuit switch may maintain or table or listof the compound connections made through the compound optical circuitswitch. This table may be parsed to identify connections made using thefailed 2^(nd)-tier switch.

In general, an optical circuit switch is oblivious to the contentcommunicated over connections made through the switch. Thus an opticalcircuit switch may be unable, of itself, to prioritize connections forrebuilding. However, priority information may be included in commandsreceived by the optical circuit switch and may be stored in the table ofconnections. Priority information may also be provided, for example by anetwork controller such as the network controller 210, at the time thefailure of the 2^(nd)-tier switch is recognized. The priorityinformation may be used at 1420 to prioritize an order in which brokenconnections will be rebuilt. When priority information is not available,the compound connections to be rebuilt may be placed in random order orany convenient order.

Each broken compound connection may be rebuilt in sequence from 1430 to1460. At 1430 a first broken compound connection may be selected inaccordance with the order determined at 1420. At 1440, a determinationmay be made whether or not a path is available to rebuild the selectedconnection. Each broken full duplex compound connection connects aninput/output port pair at a first 1^(st)-tier switch with aninput/output port pair at a second 1^(st)-tier switch. Since the twodirections of the full duplex compound connection did not necessarilypass through the same 2^(nd) tier switch, one or both of the directionsof the full duplex compound connection may need to be rebuilt. In orderto rebuild a connection, it is necessary that a path is availablebetween the first and second 1^(st)-tier switches via one of the working2^(nd) tier switches.

In a compound optical circuit switch without redundancy, such as thecompound optical circuit switches 400, 600, 700, and 800, paths torebuild broken compound connections will be available only to the extentthat some ports of other 2^(nd)-tier switches were unused prior to thefailure of the failed 2^(nd)-tier switch. If a path is available torebuild the compound connection identified at 1430, the connection maybe rebuilt at 1450. Rebuilding the compound connection at 1450 mayinclude issuing local connection commands to the appropriate 1^(st)-tierand 2^(nd)-tier switches to make and optimize the necessary connections,such as described in conjunction with actions 1260-1285 of the process1200.

In a compound optical circuit switch with redundancy, such as thecompound optical circuit switch 900, a path to rebuild any brokencompound connection is guaranteed. In such a compound optical circuitswitch, the determination at 1440 is unnecessary and the connectionselected at 1430 may be rebuilt at 1450.

After one or both directions of a compound connection are rebuilt at1450, or when a determination is made at 1440 that no path is available,the process 1400 may proceed to 1460. At 1460, a determination may bemade if there are more broken compound connections that may possibly berebuilt. When one or more broken compound connections are identified,the process 1400 may return to 1430 to select the next broken compoundconnection. The actions from 1430 to 1460 may be repeated until allbroken compound connections have been considered. When all brokencompound connections have been considered (“No” at 1460), the processmay end at 1490.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A compound optical circuit switch to provideselectable optical connections between a plurality of input ports and aplurality of output ports, comprising: two or more 1^(st)-tier switches;and one or more 2^(nd)-tier switches coupled to the 1^(st)-tierswitches, wherein each of the plurality of input ports is connected to arespective input of one of the 1^(st)-tier switches, each of theplurality of output ports is connected to a respective output of one ofthe 1^(st)-tier switches, each connection between an input port and anoutput port connected to the same 1^(st)-tier switch is made as a simpleconnection within the 1^(st)-tier switch, and each connection between aninput port and an output port connected to two different 1^(st)-tierswitches is made as a compound connection along a respective connectionpath through the 1^(st)-tier switch connected to the input port, througha selected 2^(nd)-tier switch, and through the 1^(st)-tier switchconnected to the output port.
 2. The compound optical circuit switch ofclaim 1, wherein each of the one or more 2^(nd)-tier switches has atleast some inputs connected to respective outputs of each of the two ormore 1^(st)-tier switches, and each of the one or more 2^(nd)-tierswitches has at least some outputs connected to respective inputs ofeach of the two or more 1^(st)-tier switches.
 3. The compound opticalcircuit switch of claim 1, wherein the plurality of input ports and theplurality of output ports comprise kn input ports and kn output ports,respectively, where k is a positive integer and n is an even positiveinteger, the two or more 1^(st)-tier switches comprise 2k 1^(st)-tierswitches, each of the 2k 1^(st)-tier switches configured to provideselectable optical connections between n inputs and n outputs, and theone or more 2^(nd)-tier switches comprise j 2^(nd)-tier switches, eachof the j 2^(nd)-tier switches configured to provide selectable opticalconnections between m inputs and m outputs, where j and m are positiveintegers and jm=kn.
 4. The compound optical circuit switch of claim 3,wherein n/2 inputs of each of the 1^(st)-tier switches are connectedrespectively to n/2 input ports, n/2 outputs of each of the 1^(st)-tierswitches are connected respectively to n/2 output ports, n/2j inputs ofeach of the 1^(st)-tier switches are connected to respective outputs ofeach of the 2^(nd)-tier switches, and n/2j outputs of each of the1^(st)-tier switches are connected to respective inputs of each of the2^(nd)-tier switches.
 5. The compound optical circuit switch of claim 3,wherein the n/2 input ports and n/2 output ports are combined in pairsto provide n/2 full-duplex ports.
 6. The compound optical circuit switchof claim 1, further comprising: a compound connection controllerconfigured to: receive a global connection command identifying an inputport from the plurality of input ports and an output port from theplurality of output ports to be connected through the compound switch;select a connection path from the identified input port to theidentified output port; and when the identified input port and theidentified output port are connected to the same 1^(st)-tier switch,issue a local command to that 1^(st)-tier switch to effect a simpleconnection between the identified input port and the identified outputport, or when the identified input port and the identified output portare connected to different 1^(st)-tier switches, issue local commands tothe 1^(st)-tier switch connected to the identified input port, theselected 2^(nd)-tier switch, and the 1^(st)-tier switch connected to theidentified output port to effect a compound connection.
 7. The compoundoptical circuit switch of claim 6, wherein the compound connectioncontroller is further configured to: when the identified input port andthe identified output port are connected to different 1^(st)-tierswitches, select the connection path from the input port to the outputport based, in part, on an expected insertion loss of the connectionpath.
 8. The compound optical circuit switch of claim 6, wherein thecompound connection controller comprises: an expected loss table storingan expected insertion loss for at least some of the possible connectionsbetween an input and an output of each of 1^(st)-tier switches and eachof the 2^(nd)-tier switches.
 9. The compound optical circuit switch ofclaim 8, wherein the compound connection controller is furtherconfigured to: select the connection path from the input port to theoutput port based, in part, on a insertion loss target included in theglobal connection command.
 10. The compound optical circuit switch ofclaim 8, wherein the compound connection controller is furtherconfigured to: select the connection path from the input port to theoutput port to equalize, to an extent possible, insertion losses of allcompound connections.
 11. The compound optical circuit switch of claim10, wherein the compound connection controller is further configured to:select an available connection path having an insertion loss that ismost nearly equal to an average expected insertion loss for all compoundconnections.
 12. A method of making connection through a compoundoptical circuit switch including two or more 1^(st)-tier switches andone or more 2^(nd)-tier switches, the method comprising: receiving aglobal connection command identifying an input port from a plurality ofinput ports and an output port from a plurality of output ports to beconnected; when the identified input port and the identified output portare connected to the same 1^(st)-tier switch, making a simple connectionwithin the 1^(st)-tier switch; and when the identified input port andthe identified output port are connected to two different 1^(st)-tierswitches, making a compound connection along a respective connectionpath through the 1^(st)-tier switch connected to the input port, througha selected 2^(nd)-tier switch, and through the 1^(st)-tier switchconnected to the output port.
 13. The method of claim 12, furthercomprising: selecting the connection path from the identified input portto the identified output port; and when the identified input port andthe identified output port are connected to the same 1^(st)-tier switch,issuing a local command to that 1^(st)-tier switch to effect the simpleconnection between the identified input port and the identified outputport, or when the identified input port and the identified output portare connected to different 1^(st)-tier switches, issuing local commandsto the 1^(st)-tier switch connected to the identified input port, theselected 2^(nd)-tier switch, and the 1^(st)-tier switch connected to theidentified output port to effect the compound connection.
 14. The methodof claim 13, wherein selecting the connection path further comprises:when the identified input port and the identified output port areconnected to different 1^(st)-tier switches, selecting the connectionpath from the input port to the output port based, in part, on anexpected insertion loss of the connection path.
 15. The method of claim14, further comprising: storing an expected insertion loss for at leastsome of the possible connections between an input and an output of eachof 1^(st)-tier switches and each of the 2^(nd)-tier switches in anexpect loss table.
 16. The method of claim 15, wherein selecting theconnection path further comprises: selecting the connection path fromthe input port to the output port based, in part, on an insertion losstarget included in the global connection command and the expected losstable.
 17. The method of claim 15, wherein selecting the connection pathfurther comprises: selecting the connection path from the input port tothe output port based on the expect loss table to equalize, to extentpossible, insertion losses of all compound connections.
 18. The methodof claim 17, wherein selecting the connection path further comprises:selecting an available connection path having an insertion loss that ismost nearly equal to an average expected insertion loss for all compoundconnections.